Conditions on Gliese 581 c are just right, and life there is 'indeed possible'

By Dan VerganoUSA TODAY

European astronomers announced Tuesday that they've discovered the first potentially habitable Earthlike planet outside our solar system where conditions are such that oceans, and thus life, may be possible.

The planet circles the red dwarf star Gliese 581 in a Goldilocks orbit that sets its temperature between 32 and 104 degrees Fahrenheit, neither too hot nor too cold — but just right — for liquid water to exist, according to the team led by astronomer Michel Mayor of Switzerland's Geneva Observatory.

Of the more than 200 planets detected orbiting nearby stars since 1995, most are gas giants. The new discovery, dubbed Gliese 581 c, is rocky like Earth and the first small planet detected at the right distance from its star to harbor oceans. NASA has long believed it needs to "follow the water" in its search for life elsewhere.

"Life on the planet we have discovered is indeed possible," says team member Xavier Bonfils of Portugal's Lisbon Observatory in an e-mail. "We're very excited by this discovery because we feel we are entering a new (class) of planets like Earth."

Gliese 581 is about 20.5 light-years away in the constellation Libra (one light-year is about 5.9 trillion miles), much less bright than our sun. Because the star isn't as hot, the planet, one of three circling the star, orbits within a habitable zone about 14 times closer to its star than Earth's habitable orbit around the sun. Gliese 581 c completes a "year" in only 13 days, Bonfils says.

"This is the closest planet to another Earth that has been found to date," says astronomer Alan Boss of the Carnegie Institute of Washington (D.C.), by e-mail. "The fact that it could have liquid water makes it even more fascinating and arguably the first habitable planet."

The discovery team detected the planet indirectly, by measuring back-and-forth gravitational wobbles that the orbiting planets induce in Gliese 581, a star about one-third as heavy as the sun. That method provides only a minimum mass estimate of a planet. In this case, Gliese 581 c is likely about five times heavier than Earth and 1½ times as wide. It's one of the three smallest planets yet detected orbiting a nearby star.

"We may be witnessing the start of a cosmic real estate boom," says astronomer Jill Tarter of the SETI Institute in Mountain View, Calif. SETI, the Search for Extraterrestrial Intelligence, has surveyed Gliese 581 for radio signals, without result, in its privately funded Project Phoenix effort, according to Tarter.

Gliese 581 "appears to be a nice, quiet older star that orbits placidly within the disk of the galaxy," says astronomer Margaret Turnbull of the Space Telescope Science Institute in Baltimore. "Because the star itself is so small, it makes sense that the planets orbiting it would also be small — so we should not be shocked about that. "

"Water will be the key" as to whether life could exist on Gliese 581 c, and determining, with a telescope, whether oceans exist there will be tricky, Tarter says.

Astronomer Wesley Traub of NASA's Jet Propulsion Laboratory in Pasadena, Calif., doubts Gliese 581 c is hospitable enough for life. "It is probably tidally locked to the star, like the moon to the Earth," he says. That means the star-facing side of the planet would receive boiling heat, while the far side would be frozen.

Regardless, the planet has likely rocketed to the top of the list for would-be Earth-hunters, Bonfils says. Red dwarf stars are 80 of the 100 closest stars to Earth, he adds, making the detection of more small planets orbiting red dwarfs likely.